Multicellular organisms skillfully control the proliferation and death of cells to maintain their homeostasis. Many cells are removed by cell death in the course of ontogeny. In an adult, cells constituting organs always maintain their functions while keeping a balance between their proliferation and death. Such cell death is preliminarily programmed death called “programmed cell death” and is distinguished from “accidental cell death” caused by physical and chemical factors. These two deaths are different from each other in process. More specifically, the programmed cell death is caused by a process of apoptosis, while in the accidental cell death, cells are killed via a process of necrosis.
A Fas antigen is a cell-surface protein that mediates cell death (apoptosis). Recently, a cDNA of the Fas antigen was cloned jointly by Dr. Naoto Ito, Dr. Shigekazu Nagata et al. in Osaka Bioscience Institute (Cell, Vol. 66, pp. 223-243, 1991). It was found from the structure of the cDNA thus obtained that a human Fas antigen is a transmembrane protein consisting of 319 amino acid residues and has one transmembrane region. The extracellular region of the Fas antigen is constituted by 157 amino acid residues and has a cysteine residue-rich structure. A mouse Fas antigen consists of 306 amino acid residues and has a homology of 49.3% with the human Fas antigen.
It was found that the cysteine residue-rich structure of the extracellular region in the Fas antigen is a well conserved structure recognized in a low-affinity receptor of NGF (nerve growth factor) and a receptor of TNF (tumor necrosis factor). This fact revealed that the Fas antigen is a cell-surface protein belonging to the NGF/TNF receptor family. Since many of proteins belonging to this family have their ligands in the living body, the Fas antigen is also expected to have its ligand in the living body. A molecule of a rat Fas ligand was identified by a group of Dr. Shigekazu Nagata et al. in Osaka Bioscience Institute in 1993 (Cell, Vol. 75, pp. 1169-1178, 1993), and subsequently molecules of mouse and human Fas ligands were identified by the same group (Int. Immunol., Vol. 6 No. 10, pp. 1567-1574).
It has been understood that the Fas antigen mediates a signal of “death” to cells. Besides, an anti-Fas antibody induces apoptosis against certain cells. In a mouse having 1pr (lymphoproliferation) mutation exhibiting the symptom of autoimmune disease, it has been found that the mutation exists in its Fas gene. These results suggest that the inactivation of proteins mediating apoptosis, such as the Fas antigen, causes abnormal proliferation of cells, while abnormal activation thereof causes certain inflammatory reactions.
For example, it has been reported that the expression of Fas is recognized in acquired immunodeficiency virus-infected T cells (Proc. Natl. Acad. Sci. USA, Vol. 87, pp. 9620-9624, 1990), that when an anti-Fas antibody (Jo-2) is intraperitoneally administered to mice, the mice are attacked by fulminant hepatitis (Nature, Vol. 364, pp. 806-809, 1993), that the expression of Fas is recognized in viral hepatitis (Hepatology, Vol. 19, pp. 1354-1359, 1994), and that even in autoimmune diseases, the expression of Fas is recognized in SLE (systemic lupus erythematodes) and RA (rheumatoid arthritis). These may be considered to be caused by a Fas ligand reacting with a Fas antigen. However, it takes formidable experiments to actually confirm them.
As described above, the researches of Fas antigens prove that in an immune system, a system mediating a signal of “death” works from the outside of cells. However, there has been yet no knowing whether the cell death in development and neurocytes is induced by a like signal from the outside (the system of Fas works) or programmed in cells as called programmed cell death. Its elucidation is an important problem in future.
A signal transfer mechanism for inducing apoptosis against cells, i.e., a problem that apoptosis is induced from a Fas antigen by what signal transfer mechanism, is also not elucidated. In order to exactly understand the system of Fas, it is necessary to make a ligand of the Fas (Fas ligand) and its function clear and to reconsider the system of Fas from the viewpoint of the interaction between ligand and receptor.
As described above, the gene of a Fas ligand was identified by Dr. Shigekazu Nagata et al. As a result, according to the above literature, “Cell”, it has been found that the Fas ligand is a protein consisting of 278 amino acids with a molecular weight of 31,138, and it has also been found that 4 N-glycoside-bond sites exist therein, and it is hence a glycoprotein (Cell Technology, Vol. 13 No. 8, pp. 738-744, 1994).
The report in literature by Hanabuchi et al. (Proc. Natl. Acad. Sci. USA, Vol. 91, No. 11, pp. 4930-4934, 1994) has showed that as a result of the analysis of the mechanism of lysing target cells by killer T cells via a Fas antigen, there is a possibility that the transmission of an apoptosis signal via the Fas antigen on the target cells may take part in the lysis of the target cells by CD4+ T cells (CTL) which do not express perforin. This has revealed that a Fas ligand exists on the cell surface of CD4+ CTL.
In a mouse having gld (generalized lymphoproliferative disease) mutation exhibiting the symptom of autoimmune disease, it has been found that the mutation exists in its Fas gene (Cell, Vol. 76, pp. 969-979, 1994).
However, the recognition that a Fas ligand may play an important role in vital reactions has been just gained under circumstances. As described above, the Fas ligand molecule has been just identified at present, and so the mechanism of Fas and the Fas ligand has been just started to be elucidated. In order to make this mechanism clear, analysis at the protein level (immunological analysis), or acquisition of neutralizing antibodies or the like which inhibit the binding action of Fas to the Fas ligand is essential.